Identification of ground beef-derived fatty acid inhibitors of autoinducer-2-based cell signaling

Targeting microbial quorum sensing: the next frontier to hinder bacterial driven gastrointestinal infections

Bacteria synchronize social behaviors via a cell-cell communication and interaction mechanism termed as quorum sensing (QS). QS has been extensively studied in monocultures and proved to be intensively involved in bacterial virulence and infection. Despite the role QS plays in pathogens during laboratory engineered infections has been proved, the potential functions of QS related to pathogenesis in context of microbial consortia remain poorly understood. In this review, we summarize the basic molecular mechanisms of QS, primarily focusing on pathogenic microbes driving gastrointestinal (GI) infections. We further discuss how GI pathogens disequilibrate the homeostasis of the indigenous microbial consortia, rebuild a realm dominated by pathogens, and interact with host under worsening infectious conditions via pathogen-biased QS signaling. Additionally, we present recent applications and main challenges of manipulating QS network in microbial consortia with the goal of better understanding GI bacterial sociality and facilitating novel therapies targeting bacterial infections.

Bringing Antimicrobial Strategies to a New Level: The Quorum Sensing System as a Target to Control Streptococcus suis

Streptococcus suis (S. suis) is an important zoonotic pathogen. It mainly uses quorum sensing (QS) to adapt to complex and changeable environments. QS is a universal cell-to-cell communication system that has been widely studied for its physiological functions, including the regulation of bacterial adhesion, virulence, and biofilm formation. Quorum sensing inhibitors (QSIs) are highly effective at interfering with the QS system and bacteria have trouble developing resistance to them. We review the current research status of the S. suis LuxS/AI-2 QS system and QSIs. Studies showed that by inhibiting the formation of AI-2, targeting the LuxS protein, inhibiting the expression of luxs gene can control the LuxS/AI-2 QS system of S. suis. Other potential QSIs targets are summarized, which may be preventing and treating S. suis infections, including AI-2 production, transmission, LuxS protein, blockage of AI-2 binding to receptors, AI-2-mediated QS. Since antibiotics are becoming increasingly ineffective due to the emergence of resistant bacteria, including S. suis, it is thus critical to find new antibacterial drugs with different mechanisms of action. QSIs provide hope for the development of such drugs.

Quorum sensing in human gut and food microbiomes: Significance and potential for therapeutic targeting

Human gut and food microbiomes interact during digestion. The outcome of these interactions influences the taxonomical composition and functional capacity of the resident human gut microbiome, with potential consequential impacts on health and disease. Microbe-microbe interactions between the resident and introduced microbiomes, which likely influence host colonisation, are orchestrated by environmental conditions, elements of the food matrix, host-associated factors as well as social cues from other microorganisms. Quorum sensing is one example of a social cue that allows bacterial communities to regulate genetic expression based on their respective population density and has emerged as an attractive target for therapeutic intervention. By interfering with bacterial quorum sensing, for instance, enzymatic degradation of signalling molecules (quorum quenching) or the application of quorum sensing inhibitory compounds, it may be possible to modulate the microbial composition of communities of interest without incurring negative effects associated with traditional antimicrobial approaches. In this review, we summarise and critically discuss the literature relating to quorum sensing from the perspective of the interactions between the food and human gut microbiome, providing a general overview of the current understanding of the prevalence and influence of quorum sensing in this context, and assessing the potential for therapeutic targeting of quorum sensing mechanisms.

Plant compounds and nonsteroidal anti-inflammatory drugs interfere with quorum sensing in Chromobacterium violaceum

Chromobacterium violaceum is a Gram-negative, saprophytic bacterium that can infect humans and its virulence may be regulated by quorum sensing via N-acyl homoserine lactones. A virtual screening study with plant compounds and nonsteroidal anti-inflammatory drugs for inhibition of C. violaceum quorum sensing system has been performed. In vitro evaluation was done to validate the in silico results. Molecular docking showed that phytol, margaric acid, palmitic acid, dipyrone, ketoprofen, and phenylbutazone bound to structures of CviR proteins of different C. violaceum strains. Phytol presented higher binding affinities than AHLs and furanones, recognized inducers, and inhibitors of quorum sensing, respectively. When tested in vitro, phytol at a non-inhibitory concentration was the most efficient tested compound to reduce phenotypes regulated by quorum sensing. The results indicate that in silico compound prospection to inhibit quorum sensing may be a good tool for finding alternative lead molecules.

Biofabricated Fatty Acids-Capped Silver Nanoparticles as Potential Antibacterial, Antifungal, Antibiofilm and Anticancer Agents

The current study demonstrates the synthesis of fatty acids (FAs) capped silver nanoparticles (AgNPs) using aqueous poly-herbal drug Liv52 extract (PLE) as a reducing, dispersing and stabilizing agent. The NPs were characterized by various techniques and used to investigate their potent antibacterial, antibiofilm, antifungal and anticancer activities. GC-MS analysis of PLE shows a total of 37 peaks for a variety of bio-actives compounds. Amongst them, n-hexadecanoic acid (21.95%), linoleic acid (20.45%), oleic acid (18.01%) and stearic acid (13.99%) were found predominately and most likely acted as reducing, stabilizing and encapsulation FAs in LIV-AgNPs formation. FTIR analysis of LIV-AgNPs shows some other functional bio-actives like proteins, sugars and alkenes in the soft PLE corona. The zone of inhibition was 10.0 ± 2.2-18.5 ± 1.0 mm, 10.5 ± 2.5-22.5 ± 1.5 mm and 13.7 ± 1.0-16.5 ± 1.2 against P. aeruginosa, S. aureus and C. albicans, respectively. LIV-AgNPs inhibit biofilm formation in a dose-dependent manner i.e., 54.4% ± 3.1%-10.12% ± 2.3% (S. aureus), 72.7% ± 2.2%-23.3% ± 5.2% (P. aeruginosa) and 85.4% ± 3.3%-25.6% ± 2.2% (C. albicans), and SEM analysis of treated planktonic cells and their biofilm biomass validated the fitness of LIV-AgNPs in future nanoantibiotics. In addition, as prepared FAs rich PLE capped AgNPs have also exhibited significant (p < 0.05 *) antiproliferative activity against cultured HCT-116 cells. Overall, this is a very first demonstration on employment of FAs rich PLE for the synthesis of highly dispersible, stable and uniform sized AgNPs and their antibacterial, antifungal, antibiofilm and anticancer efficacy.

Unsaturated Fatty Acids Control Biofilm Formation of Staphylococcus aureus and Other Gram-Positive Bacteria

Infections involving biofilms are difficult to treat due to increased resistances against antibiotics and the immune system. Hence, there is an urgent demand for novel drugs against biofilm infections. During our search for novel biofilm inhibitors from fungi, we isolated linoleic acid from the ascomycete Hypoxylon fragiforme which showed biofilm inhibition of several bacteria at sub-MIC concentrations. Many fatty acids possess antimicrobial activities, but their minimum inhibitory concentrations (MIC) are high and reports on biofilm interferences are scarce. We demonstrated that not only linoleic acid but several unsaturated long-chain fatty acids inhibited biofilms at sub-MIC concentrations. The antibiofilm activity exerted by long-chain fatty acids was mainly against Gram-positive bacteria, especially against Staphylococcus aureus. Micrographs of treated S. aureus biofilms revealed a reduction in the extracellular polymeric substances, pointing to a possible mode of action of fatty acids on S. aureus biofilms. The fatty acids had a strong species specificity. Poly-unsaturated fatty acids had higher activities than saturated ones, but no obvious rule could be found for the optimal length and desaturation for maximal activity. As free fatty acids are non-toxic and ubiquitous in food, they may offer a novel tool, especially in combination with antibiotics, for the control of biofilm infections.

Current advances in Vibrio harveyi quorum sensing as drug discovery targets

Vibrio harveyi is a marine bacterial pathogen which infects a wide range of marine organisms and results in severe loss. Antibiotics have been used for prophylaxis and treatment of V. harveyi infection. However, antibiotic resistance is a major public health threat to both human and animals. Therefore, there is an urgent need for novel antimicrobial agents with new modes of action. In V. harveyi, many virulence factors production and bioluminescence formation depend on its quorum sensing (QS) network. Therefore, the QS system has been widely investigated as an effective potential target for the treatment of V. harveyi infection. This perspective focuses on the quorum sensing inhibitors (QSIs) of V. harveyi QS systems (LuxM/N, LuxS/PQ, and CqsA/S) and evaluates medicinal chemistry strategies.

Life and death of liquid-infused surfaces: a review on the choice, analysis and fate of the infused liquid layer

We review the rational choice, the analysis, the depletion and the properties imparted by the liquid layer in liquid-infused surfaces – a new class of low-adhesion surface.

Antiviral activity of palmitic acid via autophagic flux inhibition in zebrafish (Danio rerio)

Fatty acids (FAs) are key elements that affect not only growth but also different immune functions, and therefore, nutrition is important for growing healthy fish. Zebrafish (Danio rerio) is a good model for assessing the beneficial effects of immunostimulants, including FAs, before applying them in aquaculture. Accordingly, this study evaluated the effects of palmitic acid (PA) treatment on different immune parameters of zebrafish and on the mortality caused by the spring viremia of carp virus (SVCV). The results suggest that PA modulates the infection outcome in vivo, which benefits zebrafish and results in reduced mortality and viral titres. The antiviral protection elicited by this FA seems to be associated with the inhibition of autophagy and is independent of other immune processes, such as neutrophil proliferation or type I interferon (IFN) activity. The use of PA as an immunostimulant at low concentrations showed great potential in the prevention of SVCV infections; therefore, this FA could help to prevent the mortality and morbidity caused by viral agents in aquacultured fish. Nevertheless, the potentially detrimental effects of suppressing autophagy in the organism should be taken into account.

Quorum Sensing Inhibitors from Marine Microorganisms and Their Synthetic Derivatives

Quorum sensing inhibitors (QSIs) present a promising alternative or potent adjuvants of conventional antibiotics for the treatment of antibiotic-resistant bacterial strains, since they could disrupt bacterial pathogenicity without imposing selective pressure involved in antibacterial treatments. This review covers a series of molecules showing quorum sensing (QS) inhibitory activity that are isolated from marine microorganisms, including bacteria, actinomycetes and fungi, and chemically synthesized based on QSIs derived from marine microorganisms. This is the first comprehensive overview of QSIs derived from marine microorganisms and their synthetic analogues with QS inhibitory activity.

Influence of Linoleic Acid on Quorum Sensing in Proteus mirabilis and Serratia marcescens

Quorum sensing (QS) is a bacterial cell density dependent mode of communication involved in regulation of virulence in pathogens including biofilm formation. Accordingly, curbing QS might prove to be an anti-virulence approach of controlling nosocomial infections caused by multi drug resistant bacteria. The report presented here documents the QS inhibitory properties of linoleic acid against Proteus mirabilis and Serratia marcescens known to cause nosocomial infections. Urease assay, prodigiosin assay, protease assay, biofilm formation assay and growth curve analysis were performed to investigate the effectiveness of linoleic acid in controlling virulence of P. mirabilis and S. marcescens. 2.5mM linoleic acid reduced the urease activity and biofilm formation to 42.11% and 11.11% respectively in P. mirabilis; and prodigiosin synthesis, protease activity and biofilm formation to 0%, 65.91% and 33.33% correspondingly in S. marcescens. Therefore, analysis of QS inhibitory behaviour of linoleic acid substantiates its use as a plausible drug for anti-virulence therapy without subjecting the bacteria to discerning force of antibiotics.

Antiquorum Sensing Activity of Seed Oils from Oleaginous Plants and Protective Effect During Challenge with Chromobacterium violaceum

Seed oils from oleaginous plants are rich in fatty acids (FAs) that play important roles in the health of the consumers. Recent studies indicate that FA also can play an important role in communication and regulation of virulence in bacteria. Nevertheless, evidence demonstrating protection against bacterial infections mediated by their quorum sensing inhibition (QSI) activity is scarce. In this study, sunflower, chia, and amaranth oils, were assayed for their QSI capacity by inhibiting violacein production and alkaline exoprotease activity of Chromobacterium violaceum. In vitro assays revealed that the oils exhibited QSI activities, whereas in vivo they delayed death of mice inoculated intraperitoneally with the bacterium. Gas chromatography coupled with mass spectrometry analysis of the oils indicated the presence of saturated FA (SAFA) and unsaturated FA as main components. Through a structure-activity relationship study of free FAs, bactericidal effect was identified mainly for polyunsaturated FAs, whereas QSI activity was restricted to SAFA of chains 12-18 carbon atoms in length. These data correlate with a possible interaction suggested by molecular docking analysis of lauric, myristic, and stearic acids with the CviR protein. Our study highlights the antiquorum sensing potential of SAFA, which may be future antivirulence therapeutic agents for the treatment of bacterial infections.

In vitro and in vivo exploration of palmitic acid from Synechococcus elongatus as an antibiofilm agent on the survival of Artemia franciscana against virulent vibrios

Biofilm formation of Vibrio spp. has been demonstrated as a potentially important mechanism contributing antibiotic treatment failure in aquaculture. In the present study, the effect of palmitic acid (PA) identified from Synechococcus elongatus was assessed for the inhibition of quorum sensing (QS) regulated biofilm formation in aquatic bacterial pathogens. The biofilm inhibitory concentration (BIC) of PA against Vibrio spp. was found to be 100µgml^-1. In this concentration, PA exhibited a significant inhibition in biofilm biomass of Vibrio harveyi MTCC 3438, V. parahaemolyticus ATCC 17802, V. vulnificus MTCC 1145 and V. alginolyticus ATCC 17749 without hindering their planktonic growth. Also, PA displayed gradual decrease in bioluminescence production of V. harveyi. The results of extracellular polymeric substances quantification, microbial adhesion to hydrocarbons and Fourier transform infrared spectroscopic (FT-IR) analyses suggested that PA positively interferes with the initial adhesion stages of biofilm formation. In addition, confocal and scanning electron microscopic analysis substantiates the antibiofilm efficacy of the PA. The transcriptomic analysis revealed the down-regulation of QS mediated response regulator genes expression in V. harveyi. Concomitantly, PA reduced the intestinal colonization of vibrios in brine shrimp larvae and thereby attenuates the biofilm assemblage and its associated virulence. In vivo studies using brine shrimp larvae manifested the reduction in adherence and virulence, which prompts further investigation about the potential of PA for the treatment of vibriosis.

Surface conditioning with Escherichia coli cell wall components can reduce biofilm formation by decreasing initial adhesion

Bacterial adhesion and biofilm formation on food processing surfaces pose major risks to human health. Non-efficient cleaning of equipment surfaces and piping can act as a conditioning layer that affects the development of a new biofilm post-disinfection. We have previously shown that surface conditioning with cell extracts could reduce biofilm formation. In the present work, we hypothesized that E. coli cell wall components could be implicated in this phenomena and therefore mannose, myristic acid and palmitic acid were tested as conditioning agents. To evaluate the effect of surface conditioning and flow topology on biofilm formation, assays were performed in agitated 96-well microtiter plates and in a parallel plate flow chamber (PPFC), both operated at the same average wall shear stress (0.07 Pa) as determined by computational fluid dynamics (CFD). It was observed that when the 96-well microtiter plate and the PPFC were used to form biofilms at the same shear stress, similar results were obtained. This shows that the referred hydrodynamic feature may be a good scale-up parameter from high-throughput platforms to larger scale flow cell systems as the PPFC used in this study. Mannose did not have any effect on E. coli biofilm formation, but myristic and palmitic acid inhibited biofilm development by decreasing cell adhesion (in about 50%). These results support the idea that in food processing equipment where biofilm formation is not critical below a certain threshold, bacterial lysis and adsorption of cell components to the surface may reduce biofilm buildup and extend the operational time.

Novel reporter for identification of interference with acyl homoserine lactone and autoinducer-2 quorum sensing

Two reporter strains were established to identify novel biomolecules interfering with bacterial communication (quorum sensing [QS]). The basic design of these Escherichia coli-based systems comprises a gene encoding a lethal protein fused to promoters induced in the presence of QS signal molecules. Consequently, these E. coli strains are unable to grow in the presence of the respective QS signal molecules unless a nontoxic QS-interfering compound is present. The first reporter strain designed to detect autoinducer-2 (AI-2)-interfering activities (AI2-QQ.1) contained the E. coli ccdB lethal gene under the control of the E. coli lsrA promoter. The second reporter strain (AI1-QQ.1) contained the Vibrio fischeri luxI promoter fused to the ccdB gene to detect interference with acyl-homoserine lactones. Bacteria isolated from the surfaces of several marine eukarya were screened for quorum- quenching (QQ) activities using the established reporter systems AI1-QQ.1 and AI2-QQ.1. Out of 34 isolates, two interfered with acylated homoserine lactone (AHL) signaling, five interfered with AI-2 QS signaling, and 10 were demonstrated to interfere with both signal molecules. Open reading frames (ORFs) conferring QQ activity were identified for three selected isolates (Photobacterium sp., Pseudoalteromonas sp., and Vibrio parahaemolyticus). Evaluation of the respective heterologously expressed and purified QQ proteins confirmed their ability to interfere with the AHL and AI-2 signaling processes.

Inhibition of bacterial quorum sensing by extracts from aquatic fungi: first report from marine endophytes

In our search for quorum-sensing (QS) disrupting molecules, 75 fungal isolates were recovered from reef organisms (endophytes), saline lakes and mangrove rhizosphere. Their QS inhibitory activity was evaluated in Chromobacterium violaceum CVO26. Four strains of endophytic fungi stood out for their potent activity at concentrations from 500 to 50 μg mL⁻¹. The molecular characterization, based on the internal transcribed spacer (ITS) region sequences (ITS1, 5.8S and ITS2) between the rRNA of 18S and 28S, identified these strains as belonging to four genera: Sarocladium (LAEE06), Fusarium (LAEE13), Epicoccum (LAEE14), and Khuskia (LAEE21). Interestingly, three came from coral species and two of them came from the same organism, the coral Diploria strigosa. Metabolic profiles obtained by Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS) suggest that a combination of fungal secondary metabolites and fatty acids could be the responsible for the observed activities. The LC-HRMS analysis also revealed the presence of potentially new secondary metabolites. This is, to the best of our knowledge, the first report of QS inhibition by marine endophytic fungi.

Intestinal microbiome and digoxin inactivation: meal plan for digoxin users?

There is an increasing interest in the role of intestinal microbiome in human diseases and therapeutic agents' bioavailability, activity and toxicity. Epidemiological data show that the bioavailability of digoxin, a widely used agent for heart disease, varies among individuals. The inactivation of digoxin was found when it was incubated with gut bacterium Eggerthella lenta in vitro decades ago. However, the underlying mechanisms of digoxin inactivation are still unclear. A recent study using animal models uncovered this mystery, which suggested that arginine supplements might be a potential intervention in increasing digoxin activity by inhibiting the expression of cardiac glycoside reductase gene operons that inactivated digoxin. This perspective summarizes the connections among the intestinal microbiome, the digoxin inactivation, the metabolism of arginine. We also discuss several issues yet to be addressed in the future, making better strategies in the application of dietary arginine supplements for digoxin users.

Small molecule inhibitors of AI-2 signaling in bacteria: state-of-the-art and future perspectives for anti-quorum sensing agents

Bacteria respond to different small molecules that are produced by other neighboring bacteria. These molecules, called autoinducers, are classified as intraspecies (i.e., molecules produced and perceived by the same bacterial species) or interspecies (molecules that are produced and sensed between different bacterial species). AI-2 has been proposed as an interspecies autoinducer and has been shown to regulate different bacterial physiology as well as affect virulence factor production and biofilm formation in some bacteria, including bacteria of clinical relevance. Several groups have embarked on the development of small molecules that could be used to perturb AI-2 signaling in bacteria, with the ultimate goal that these molecules could be used to inhibit bacterial virulence and biofilm formation. Additionally, these molecules have the potential to be used in synthetic biology applications whereby these small molecules are used as inputs to switch on and off AI-2 receptors. In this review, we highlight the state-of-the-art in the development of small molecules that perturb AI-2 signaling in bacteria and offer our perspective on the future development and applications of these classes of molecules.

The apparent quorum-sensing inhibitory activity of pyrogallol is a side effect of peroxide production

There currently is more and more interest in the use of natural products, such as tea polyphenols, as therapeutic agents. The polyphenol compound pyrogallol has been reported before to inhibit quorum-sensing-regulated bioluminescence in Vibrio harveyi. Here, we report that the addition of 10 mg · liter(-1) pyrogallol protects both brine shrimp (Artemia franciscana) and giant river prawn (Macrobrachium rosenbergii) larvae from pathogenic Vibrio harveyi, whereas the compound showed relatively low toxicity (therapeutic index of 10). We further demonstrate that the apparent quorum-sensing-disrupting activity is a side effect of the peroxide-producing activity of this compound rather than true quorum-sensing inhibition. Our results emphasize that verification of minor toxic effects by using sensitive methods and the use of appropriate controls are essential when characterizing compounds as being able to disrupt quorum sensing.

Quorum sensing in the context of food microbiology

Food spoilage may be defined as a process that renders a product undesirable or unacceptable for consumption and is the outcome of the biochemical activity of a microbial community that eventually dominates according to the prevailing ecological determinants. Although limited information are reported, this activity has been attributed to quorum sensing (QS). Consequently, the potential role of cell-to-cell communication in food spoilage and food safety should be more extensively elucidated. Such information would be helpful in designing approaches for manipulating these communication systems, thereby reducing or preventing, for instance, spoilage reactions or even controlling the expression of virulence factors. Due to the many reports in the literature on the fundamental features of QS, e.g., chemistry and definitions of QS compounds, in this minireview, we only allude to the types and chemistry of QS signaling molecules per se and to the (bioassay-based) methods of their detection and quantification, avoiding extensive documentation. Conversely, we attempt to provide insights into (i) the role of QS in food spoilage, (ii) the factors that may quench the activity of QS in foods and review the potential QS inhibitors that might "mislead" the bacterial coordination of spoilage activities and thus may be used as biopreservatives, and (iii) the future experimental approaches that need to be undertaken in order to explore the "gray" or "black" areas of QS, increase our understanding of how QS affects microbial behavior in foods, and assist in finding answers as to how we can exploit QS for the benefit of food preservation and food safety.

Autoinducer-2-like activity in lactic acid bacteria isolated from minced beef packaged under modified atmospheres

Fifteen fingerprints (assigned to Leuconostoc spp., Leuconostoc mesenteroides, Weissella viridescens, Leuconostoc citreum, and Lactobacillus sakei) of 89 lactic acid bacteria (LAB) isolated from minced beef stored under modified atmospheres at various temperatures were screened for their ability to exhibit autoinducer-2 (AI-2)-like activity under certain growth conditions. Cellfree meat extracts (CFME) were collected at the same time as the LAB isolates and tested for the presence of AI-2-like molecules. All bioassays were conducted using the Vibrio harveyi BAA-1117 (sensor 1(-), sensor 2(+)) biosensor strain. The possible inhibitory effect of meat extracts on the activity of the biosensor strain was also evaluated. AI-2-like activity was observed for Leuconostoc spp. isolates, but none of the L. sakei strains produced detectable AI-2-like activity. The AI-2-like activity was evident mainly associated with the Leuconostoc sp. B 233 strain, which was the dominant isolate recovered from storage at 10 and 15°C and at the initial and middle stages of storage at chill temperatures (0 and 5°C). The tested CFME samples displayed low AI-2-like activity and inhibited AI-2 activity regardless of the indigenous bacterial populations. The LAB isolated during meat spoilage exhibited AI-2-like activity, whereas the LAB strains retrieved depended on storage time and temperature. The production of AI-2-like molecules may affect the dominance of different bacterial strains during storage. The results provide a basis for further research concerning the effect of storage temperature on the expression of genes encoding AI-2 activity and on the diversity of the ephemeral bacterial population.

Detection of autoinducer (AI-2)-like activity in food samples

The contamination, survival, and possible foodborne disease outbreaks are major issues confronting the food industry. However, from a microbial perspective, any food whether natural or processed is just another environmental niche that is available for colonization. Quorum sensing or cell-cell communication is a process by which microorganisms are thought to communicate with each other using a variety of small molecules termed autoinducers. The autoinducer AI-2 is thought to be a universal signaling molecule due to its ability to modulate the gene expression of a number of different bacterial species and genera. Pathogens such as Pseudomonas aeruginosa, Aeromonas hydrophila, Vibrio anguillarum, Streptococcus sp., and Burkholderia cepacia form biofilms on a variety of man-made and natural surfaces using cell-cell mechanisms. It is important to detect and study autoinducers and their activities in foods, since a better understanding of these molecules in food and food ingredients may help in designing new approaches to thwart microbial persistence and biofilm formation. The autoinducer AI-2 is thought to be involved in microbial attachment and biofilm formation leading to food spoilage. To better understand microbial cell-cell signaling in foods especially as it relates to pathogen persistence, biofilm formation, and food spoilage, methods to process, extract, and purify autoinducer molecules need to be developed. This chapter details methods to process food samples to obtain cell-free supernatants (CFS), which could subsequently be tested for the presence of AI-2 or "AI-2-like activity" in the extracted CFS using autoinducer bioassays. Additionally, the method of synthesizing AI-2 in the laboratory is also provided. The methods that are presented in this chapter are based on previously published research articles from the authors' laboratory.

Cross species quorum quenching using a native AI-2 processing enzyme

Bacterial quorum sensing (QS) is a cell-cell communication process, mediated by signaling molecules, that alters various phenotypes including pathogenicity. Methods to interrupt these communication networks are being pursued as next generation antimicrobials. We present a technique for interrupting communication among bacteria that exploits their native and highly specific machinery for processing the signaling molecules themselves. Specifically, our approach is to bring native intracellular signal processing mechanisms to the extracellular surroundings and "quench" crosstalk among a variety of strains. In this study, the QS system based on the interspecies signaling molecule autoinducer-2 (AI-2) is targeted because of its prevalence among prokaryotes (it functions in over 80 bacterial species). We demonstrate that the Escherichia coli AI-2 kinase, LsrK, can phosphorylate AI-2 in vitro, and when LsrK-treated AI-2 is added ex vivo to E. coli populations, the native QS response is significantly reduced. Further, LsrK-mediated degradation of AI-2 attenuates the QS response among Salmonella typhimurium and Vibrio harveyi even though the AI-2 signal transduction mechanisms and the phenotypic responses are species-specific. Analogous results are obtained from a synthetic ecosystem where three species of bacteria (enteric and marine) are co-cultured. Finally, the addition of LsrK and ATP to growing co-cultures of E. coli and S. typhimurium exhibits significantly reduced native "cross-talk" that ordinarily exists among and between species in an ecosystem. We believe this nature-inspired enzymatic approach for quenching QS systems will spawn new methods for controlling cell phenotype and potentially open new avenues for controlling bacterial pathogenicity.

Insights on Escherichia coli biofilm formation and inhibition from whole-transcriptome profiling

Biofilms transform independent cells into specialized cell communities. Here are presented some insights into biofilm formation ascertained with the best-characterized strain, Escherichia coli. Investigations of biofilm formation and inhibition with this strain using whole-transcriptome profiling coupled to phenotypic assays, in vivo DNA binding studies and isogenic mutants have led to discoveries related to the role of stress, to the role of intra- and interspecies cell signalling, to the impact of the environment on cell signalling, to biofilm inhibition by manipulating cell signalling, to the role of toxin/antitoxin genes in biofilm formation, and to the role of small RNAs on biofilm formation and dispersal. Hence, E. coli is an excellent resource for determining paradigms in biofilm formation and biofilm inhibition.

Mini-review: quorum sensing in the marine environment and its relationship to biofouling

Bacterial quorum sensing (QS) is a cell-cell communication and gene regulatory mechanism that allows bacteria to coordinate swarming, biofilm formation, stress resistance, and production of toxins and secondary metabolites in response to threshold concentrations of QS signals that accumulate within a diffusion-limited environment. This review focuses on the role of QS signaling and QS inhibition in marine bacteria by compounds derived from marine organisms. Since the formation of a biofilm is considered to be an initial step in the development of fouling, direct and indirect effects of QS signals and inhibitors on the process of marine biofouling are discussed. Directions for future investigations and QS-related biotechnological applications are highlighted.

Insights into the role of quorum sensing in food spoilage

Food spoilage is a consequence of the degrading enzymatic activity of some food-associated bacteria. Several proteolytic, lipolytic, chitinolytic, and pectinolytic activities associated with the deterioration of goods are regulated by quorum sensing, suggesting a potential role of such cell-to-cell communication in food spoilage. Here we review quorum sensing signaling molecules and methods of their detection and quantification, and we provide insights into the role of quorum sensing in food spoilage and address potential quorum sensing inhibitors that might be used as biopreservatives.